Communications headsets can be used in a diversity of applications, and are particularly effective for use with mobile communications devices such as cellular telephones. The use of communications headsets with mobile communications devices depends heavily on the headsets' ability to provide consistently high transmit signal quality, of which one important measure is the signal-to-noise ratio (i.e. ratio between the amount of signals associated with the desired acoustic source such as the user's mouth and those from ambient noise). Hence, it is desirable for communications headsets to include certain mechanisms that provide a high signal-to-noise ratio when the headsets are used in noisy environments. It is particularly advantageous to be able to reduce the obscuring effect of ambient noise in the transmit signals when the headsets are used outdoors.
Previous noise reduction designs often involve complicated electrical circuitry, which are both delicate and prone to errors. For example, noise-canceling microphones, i.e., microphones that are more sensitive to sound waves approaching in certain directions relative to the others, have been designed for use in noisy environments. These microphones are constructed such that both sides of their diaphragms are exposed to sound waves, and reduce the noise content of the transmit signals, thereby increasing the signal-to-noise ratio. Another class of solutions involves the use of sophisticated signal processing techniques to reduce the level of noise content in the transmit signals. Both types of solutions have their deficiencies.
Noise-canceling microphones are typically placed at the end of a long boom so that it can be aimed at to the user's mouth. When used in a noisy environment, a noise-canceling microphone can increase the signal-to-noise ratio of the output signals for two reasons. First, provided the microphone is positioned to aim at the user's mouth, sound waves from the user's voice approach the microphone in or near the direction of maximum sensitivity. The ambient noise, on the other hand, is usually diffuse and approaches from many different directions. Thus, only a small portion of the noise approaches the microphone in the directions of high sensitivity. Even if the noise source is non-diffuse, i.e., the noise originates from one or a few specific directions, there is a high probability that a large portion of the noise approaches from directions in which the microphone is relatively insensitive. The second reason for the increase in signal-to-noise ratio relates to a phenomenon known as “proximity effect.” In essence, the proximity effect relates to directional microphones responding strongly to sound waves from close-by sources. This is because, by virtue of the curvature of the wave fronts of sound originated from a small, close-by source, the amplitude difference between the arrivals of the waves to the front and to the rear of the microphone's diaphragm becomes significant, particularly at low frequencies. The noise-canceling microphone is therefore more sensitive to the user's voice than the ambient noise from faraway sources.
The advantages of a noise-canceling microphone can only be realized if the acoustic sensing point is close to the user's mouth and appropriately positioned (e.g., in front of rather than behind the cheek bone). To satisfy these conditions generally requires a sufficiently long boom that provides the user with enough flexibility to aim the microphone towards the user's mouth. In addition, the superior performance of a noise-canceling microphone depends largely on the assumption that the noise is diffuse or that it approaches from an angle outside the maximum sensitivity range of angles associated with the microphone, which is not always valid. Moreover, noise-canceling microphones are known to be more susceptible to wind noise than omni-directional microphone because of the turbulence in pressure resulting from wind blowing on the microphone. In fact, as the directivity factor of a noise-canceling microphone is increased, so does the ratio of wind noise sensitivity to voice sensitivity.
Long booms, which place the acoustic sensing point near the user's mouth, as required for effective noise canceling, are not always desirable in communications headsets. Headsets with short booms or no booms at all are sometimes appealing because of their unobtrusive and stylish appearance and easy stowability. This is particularly true to users of portable communication applications such as mobile phones. It is therefore desirable that communications headsets can be designed with multiple modes of operation, including at least a mode featuring a long boom extending near the user's mouth to communicate in noisy environments, and a compact mode that provides convenience when noise is not a problem.
There are other ways to reduce the undesirable effect of ambient noise in the transmit signals that employ signal processing techniques. One such technique is voice expansion, which is a form of dynamic signal processing that dynamically adjusts the amplification gain (i.e., the ratio between the levels of the amplified output signals and the raw electrical signals as converted by the microphone from acoustic signals before the amplification) as a function of the transmit signal level. Hence, when a communications headset equipped with a voice expansion mechanism is used in a noisy environment, voice expansion serves to reduce the level of output signals, including both the signals originating from the desired source and the ambient noise, when the signal level is low.
A related problem with allowing conventional communications headsets to operate in multiple modes with different boom lengths is that of sound distortion in the audio transmission. The limited dynamic range of telephone lines may result in distortion in the transmission. In addition, noise-canceling microphones have different frequency response curves for differently distanced acoustic sources due to the proximity effect. Further, even if an omni-directional microphone is used, there may still be a shift in the sound spectrum associated with any change in the location of the acoustic sensing point relative to the user's mouth. This is because high frequencies are attenuated more than low frequencies when the sound is traveling in the air.
Accordingly, it is desirable to provide a communications headset that operates in multiple modes in different boom lengths with a mechanism to adjust for optimal voice sensitivity in each mode, which is simple and inexpensive to implement.